Coal liquefaction processes have been developed for converting coal to a liquid fuel product. For example, U.S. Pat. No. 3,884,794 for Bull et al discloses a solvent refined coal process for producing reduced or low ash hydrocarbonaceous solid fuel and hydrocarbonaceous distillate liquid fuel from ash-containing raw feed coal in which a slurry of feed coal and recycle solvent is passed through a preheater and dissolver in sequence in the presence of hydrogen, solvent and recycled coal minerals, which increase the liquid product yield.
A portion of the distillate liquid produced in the coal liquefaction process is separated as a raw naphtha fraction. When it has been attempted to catalytically hydrotreat the naphtha fraction, polymer forming impurities produce a polymeric deposit in various parts of the system resulting in plugging of catalyst beds, process lines, heat exchangers and various other parts of the equipment.
The use of a conventional palladium catalyst-containing guard bed to hydrogenate such polymer forming impurities in the raw naphtha stream results in saturation and removal of olefins and diolefins, but such technique does not prevent significant polymer deposits from forming when the naphtha fraction is subjected to hydrotreating.
It would be highly desirable to provide a system for prevention and removal of polymer-forming impurities while heating and vaporizing the raw naphtha fractions so as to permit hydrotreating of the naphtha without significant polymer deposition and plugging.